These graphs were taken from my diagnostics computer when I was conducting a cylinder drop test on a Mercury Verado, in-line six cylinder, 2.6 liter, supercharged 250 HP outboard engine.
The graph to the right with the significant drop is of a normal engine when I used my diagnostics computer to shut down the ignition on one cylinder while the engine was running. This test is conducted to test each cylinder’s power compared to the rest of the cylinders. This test typically works best with a slight load on the engine to bring out the difference in power when dropping each cylinder.
The diagnostics computer connects to the engine’s EMM (engine’s computer) and through my diagnostics computer I give commands to the EMM to drop each cylinder one by one. As this is done, when a faulty or dead cylinder is detected no change (or very little change) will be detected like on the graph (seen below). I will typically use an inductive clamp DVOM to verify the bad cylinder and to see the secondary voltage being produced by the ignition coil.
In sea trials, the dead cylinder can hide and the engines can still reach their rated wide open throttle range. This is why having a marine surveyor that conducts this testing is so important. Missing a dead cylinder can cost a potential buyer hundreds if not thousands of dollars after he / she has purchased the vessel and the surveyor (who does not do this testing or have any factory engine training) has stated in the report that the engines were in good working order and that they achieved their rated WOT range.
I took a video of a cylinder drop test on a pair of Mercury Optimax engines that had a dead ignition coil on the starboard engine. The video is at the link below:
Come see more about my marine engine survey services at:
Capt. John Banister, AMS®
Suenos Azules Marine Surveying and Consulting
4521 PGA Boulevard, Suite 461
Palm Beach Gardens, Florida 33418
SAMS® Accredited Marine Surveyor
ABYC® Standards Accredited
Yamaha Certified Outboard Marine Technician
Mercury / MerCruiser Certified Technician
Honda Certified Outboard Engine Technician
USPAP® Certificate on Appraisal Standards
ITC® Certified Level II Thermographer
USCG Licensed Master Captain
Member SAMS®, ABYC®, IAMI®, & NFPA®